In general, sealed roller bearings, with high sealability, prevent ingress of rain water or dusts into the inside of the bearing. The bearings are used, for example, as bearings for the suspension of automobiles. This is due to the fact that these bearings are usually exposed to severe conditions which contain muddy water, dusts etc. On the other hand, such a rolling bearing is required to have a small rotational torque since a large rotational torque on the bearing causes adverse influence to bearing temperature rise and fuel consumption. Since the sliding resistance of the seals is a major factor that influences an increase in the rotational torque of the bearing, it is desirous to provide a sealed rolling bearing that has not only a high sealability but a small sliding resistance.
One representative example of such a bearing for an automobile is described with reference to FIG. 1. FIG. 1 shows a first embodiment of the present disclosure. This bearing is used for a driving wheel of an automobile. The bearing includes an outer member integrally formed with a body mounting flange on its outer circumference. The flange is adapted to be mounted on a body (not shown) of the automobile. The outer member is also formed with double row outer raceway surfaces on its inner circumferential surface. A wheel hub has an integrally formed wheel mounting flange on one of its end to mount a wheel (not shown). One inner raceway surface is formed on the outer circumferential surface of the wheel hub. The inner raceway surface is arranged opposite to one of the double row outer raceway surfaces. A cylindrical portion, of a smaller diameter, axially extends from the inner raceway surface. The cylinder portion includes serrations formed on its inner circumferential surface to transmit torque. An inner ring is adapted to be fitted on the cylindrical portion and the inner ring includes the other inner raceway surfaces formed on its outer circumferential surface.
Double row rolling elements (balls) are arranged between the double row outer and inner raceway surfaces. The balls are freely rollably held by cages. Sealing devices are arranged at both ends of the outer member within an annular space formed by an inner member (including the wheel hub and the inner ring) and the outer member. The sealing devices prevent leakage of grease contained within the bearing as well as ingress of rain water or dusts into the inside of the bearing.
The sealing device of the inboard side, arranged between the outer member and the inner ring, includes a sealing ring with a metal core. The metal core has a substantially L-shaped cross-section and is fit into the outer member. A sealing member is integrally adhered, via vulcanized adhesion, onto the metal core. A slinger, having a similar L-shaped cross-section, is fit onto the inner ring. The sealing member is made of an elastic material, such as rubber, and has three sealing lips, an outer sealing lip, a middle sealing lip and an inner sealing lip. The tip edge of the outer sealing lip is in sliding contact with an inner side of an upstanding portion of the slinger. The tip edges of the middle sealing lip and the inner sealing lip are in sliding contact with a cylindrical portion of the slinger.
The sealing device, on the outboard side, has an annular metal core and a sealing member integrally adhered, via vulcanized adhesion, on the metal core 26. The sealing member is made of elastic material, such as rubber, and has three sealing lips. The tip edges of the three sealing lips are in direct sealing contact with the surface of the wheel hub.
In the sealing devices of the bearing for a wheel of a vehicle of the prior art, the slinger of the sealing device on the inboard side has a surface roughness of a sliding surface on which the sealing lips sliding contact which is limited to a value at the center line average height (Ra) at 0.3 μm or less and to a value at the maximum height (Ry) at 1.2 μm or less. Thus, foreign materials cannot easily enter into the inside of the bearing. This is due to the difficulty to generate a small gap at the maximum height portions and damages which would be otherwise caused at rolling contact portions by change in the properties of the grease (see Japanese Laid-open Patent Publication No. 184897/2003).
However, in order to limit the surface roughness of a sliding surface to which the sealing lips sliding contact to the value at the center line average height (Ra) at 0.3 μm or less and to the value at the maximum height (Ry) at 1.2 μm or less, it is necessary, previously, to press sheet members having the target surface roughness or to carry out lapping of the surfaces of sheet members after the pressing process. In fact, since availability of the plate member with such a target surface roughness is difficult, it is state of art, in view of its manufacturing cost, to carry out lapping of the surfaces of the sheet members after the pressing process.
Such a lapping of the surface of the slinger makes its handling very difficult, increases the number of processing steps, and further causes deformation of the surface of the slinger. Such a deformation of the sliding surface of the slinger causes variation of the interference of the sealing member. This reduces the following ability of the sealing lips and accordingly reduces the sealing of the bearing.